Geneva Mechanisms are widely used in motion picture film projectors to intermittently advance film through a film gate having a projection aperture. The film is moved or advanced by a Geneva Mechanism (also known as a "Maltese Cross") until an image frame is in alignment with the projection aperture. The film is then held stationary for a discrete time period during which light is passed through the aperture, film frame, projection lens, and onto a screen. This intermittent frame-by-frame motion of the film is enabled by the Geneva Mechanism, which comprises one portion, the driver, which rotates continuously, and which causes intermittent rotation of a second portion, the star wheel. In a motion picture projector the star wheel shares its central shaft with a sprocket, the teeth of which are engaged with perforations in the film. Therefore, when the driver moves the star wheel, both the star wheel and the film experience a resulting intermittent motion. Other mechanisms, including servo motors and a Mitchell Movement, have been used to drive film through a gate in an intermittent manner, but the Geneva Mechanism has proven itself over the past century to be particularly well suited to accurately drive a load (the film) in an intermittent fashion when the time allowed for motion of the load is minimal. For example, motion picture film is typically projected at a rate of 24 frames per second, such that a new film frame is positioned in the projection aperture every 1/24 second, or approximately 42 ms. The typical projector Geneva Mechanism moves a film frame into the projection aperture with an indexing time of one-fourth of the frame period, or approximately 10.5 ms.
During essentially all of this indexing time, a shutter must block the light incident to the film to prevent the appearance of "travel ghost" (smear of the image caused by film motion). The projectable frame time, which would appear to be approximately three-fourths of the total frame period, is further reduced to only approximately one-half of the total frame period because the typical motion picture projector employs a two-bladed shutter, which causes two blanking periods per frame of the film, in order to raise the apparent frame rate to 48 frames per second, and thereby greatly reduce the apparent flicker perceived by the human eye. Furthermore, it is necessary for these two shutter intervals to be nearly equal in duration in order to limit perceived flicker. Therefore, since one blanking period must be approximately one-fourth of the frame period in order to blank the projected image as the film moves, the other blanking period must be of essentially the same duration.
A basic Geneva Mechanism, as is commonly employed in motion picture projectors, is described in U.S. Pat. No. 1,774,789. In general, a Geneva Mechanism is used to convert uniform rotary motion to incremental rotary motion. Typically, such a mechanism includes a star wheel having a plurality of radially extending straight slots spaced equally around the periphery of the star. Interposed between these slots are concave cam guide surfaces, which, like the slots, are uniformly dimensioned and arranged. A driver component, comprised of a restraining cam, a drive arm extending from the base of the cam, and a drive pin near the far end of the drive arm, is employed for indexing the star wheel. The restraining cam has a side cam surface, which is convex and configured to interact with the concave cam guide surfaces of the star wheel. The close contact of this convex cam surface to the concave cam guide surfaces restrains the star wheel from experiencing rotary motion except during the periods in which the star wheel is driven by the drive pin. The star wheel is thus restrained intermittently, and in a manner such that the straight slots sequentially receive the drive pin.
The number of slots radially disposed around a Geneva Mechanism's star wheel is variable, and may be any whole number greater than 2. As the number of straight slots is changed, specific features of the mechanism such as component sizes, the speed and duration of the intermittent motion, and the forces or loads applied to the drive pin and star wheel, and to the load (film) all vary as well. For example, the Geneva Mechanisms used in motion picture projectors (which vary some in detail depending on the projector) typically use a star wheel with four equally spaced straight-sided slots. Such star wheels with four straight slots are engaged with the driver pin for 90.degree. of a 360.degree. revolution of the driver, producing the intermittent motion. Thus, in the case of film projected at 24 frames per second, the star wheel and film experience movement during an indexing time of only approximately 10.5 ms of the approximately 42 ms available time per frame. By comparison, a star wheel with three straight slots will also experience intermittent motion once per revolution of the cam, but the engagement of the star wheel with the driver pin occurs over only 60.degree. of the 360.degree. revolution of the driver. If used in a motion picture projector at 24 frames per second, a star wheel with three slots would utilize an indexing time of approximately 7 ms per frame to move a film frame into the projecting aperture. While a three slot star wheel would thus decrease the time required to move the film and thereby increase the available projection time, the acceleration forces applied to the drive pin, slots, and the load (the film and film perforations) are greatly increased over those of a four-slot mechanism, making the three-slot mechanism undesirable for use in a projector. On the other hand, a star wheel with five straight slots will have any one slot engaged with the driver pin over 118.degree. of the 360.degree. revolution of the driver, for an indexing time of approximately 14 ms versus the approximately 42 ms available time per frame. This is not desirable in a projector, as the light efficiency to the screen would be significantly reduced when compared with those of a four-slot mechanism, and a more powerful lamp would be needed to obtain the same screen luminance. However, the acceleration forces on the drive pin, star wheel, and load (film perforations) would be reduced relative to a three or four straight slot star wheel.
Accordingly, it would be very advantageous to find a geometry for a Geneva Mechanism in which the indexing time is reduced, and therefore the projectable time available per frame is increased, without necessarily incurring a substantial increase in the acceleration forces applied to the drive pin, star wheel slots, and the load (the film and film perforations). U.S. Pat. No. 1,801,969 proposes to solve this problem by altering the slots of the star wheel to have curved surfaces. Thus, as the drive pin moves in engagement with these curved slots, the acceleration and velocity experienced by both the star wheel and the load (film) are altered, and the indexing time is greatly reduced when compared with that of a star wheel with an identical number of straight slots operating at the same frame rate. However, the designs for the curved star wheel slots described in U.S. Pat. No. 1,801,969 fail to adequately provide for the forces applied both to the drive pin and to the star wheel slot surfaces with which the pin is in contact. As a result, a drive pin and/or slot surfaces manufactured per U.S. Pat. No. 1,801,969 would quickly experience overloading failure during operation. Furthermore, the curved star wheel slot geometry as described in U.S. Pat. No. 1,801,969 also fails to provide sufficient slot width at the mouth of the slot for the drive pin to enter and exit the slot without very heavy interference.
It should therefore readily be appreciated that there currently exists a need for an improved Geneva Mechanism, which includes a driver with a cam and a star wheel; in which a load can be advanced intermittently with a reduced indexing time required to experience the intermittent motion, while the load forces applied to the star wheel, drive pin, and the load are all controlled, without sacrificing any of the advantages normally attendant to the use of a Geneva Mechanism. It should also be apparent, that other improved configurations for Geneva Mechanisms, in which the indexing time required to experience the intermittent motion can be reduced while the forces on the load driven by the star wheel and on the drive pin are also reduced, would likewise be advantageous. Likewise, it should be apparent that yet other improved configurations for Geneva Mechanisms, in which the indexing time is effectively unchanged, but where the load forces applied to the star wheel, drive pin, and load are substantially reduced, would also have value. Finally, it should be readily apparent that such improved Geneva Mechanisms could be employed in devices generally, and in motion picture projectors particularly, with an advantageous affect.